1. Field of the Invention
This invention relates to a semiconductor laser device and more specifically, to a buried-heterostructure laser device.
2. Description of the Prior Art
Semiconductor laser devices are generally of a double-hetero structure laser (generally referred to as a "DH laser") that has high optical confinement in the junction region. This DH laser is constructed by so forming semiconductor layers as to interpose an active layer, in which laser oscillation or light modulation is effected, between them, which semiconductor layers have a refractive index lower than that of the active layer and a greater forbidden band gap.
The DH laser has varying refractive indices in the longitudinal direction or in the direction of depth, but no difference of the refractive index occurs in the transverse direction. Accordingly a buried-hetero structure laser (generally referred to as a "BH laser") has been proposed in which a belt-like mesa is formed on a crystal surface which is parallel to the hetero-junction surface and a semiconductor layer of a low refractive index is formed in this mesa region.
Of the BH laser, a detailed report is made in "Journal of Applied Physics", Vol. 45, November No. 11, 1975, pp. 4899-4906, and the like, by way of example. U.S. Pat. No. 4,121,177 can be cited also as one of the prior art references. As to the buried-stripe double-heterostructure GaInAsP/InP diode laser, reference is to be had to "Applied Physics Letters", Vol. 30, No. 8, 15 Apr., 1977, pp. 429-431, "Lectures of No. 40th Japan Society of Applied Physics (1979)", pp. 186, 1p-N-11, entitled "The Characteristics of 1.3 .mu.m InGaAsP/InP Buried-Heterostructure (BH) Laser", and so forth.